Sensors for measuring atomic forces have been in use in a wide variety of measuring instruments, chiefly scanning microscopes, since the discovery of the scanning tunnel microscope by Binnig and Rohrer at the IBM Ruchlikon Research Laboratory near Zurich.
In the scanning tunnel microscope, a tiny needle--in the case of Binnig and Rohrer a tungsten needle--is directed near a surface to be examined (ca. 1 nm) such that, when a voltage is applied, the electrons from the tungsten needle can bridge the gap to the surface being examined by virtue of the tunnel effect.
A surface can then be imaged by moving the needle in the equidistant plane (x-y plane), and the distance of the needle tip to the surface (z direction) is then controlled using a positioning element within a closed measuring loop such that the measured electron current is constant.
The advantage of this new type of microscopy lies primarily in the fact that, in contrast to the conventional "distant field" microscope, in which the resolution according to Abbe is limited to the half-wavelength of the radiation employed, the scanning tunnel and scanning force microscopes are "near field" microscopes, whose resolution is not limited by the "wave length" of an employed interaction.
The resolution in the z direction lies on the order of one picometer, while the resolution in the x-y direction, which depends primarily on the accuracy of the needle line tracking, is ca. 0.2 nm.
As is known, however, the scanning tunnel microscope is beset with a number of disadvantages. On the one hand, measurement with respect to the surface structure is not possible in the conventional sense of an image area, but rather only with respect to the area of equal tunnel probabilities of the needle tip electrons towards the surface under examination. With materials whose surface contains only similar atoms, the image of the electron tunnel probability corresponds approximately to the optical representation of the surface. If the surface comprises a material with different atoms (e.g., a multi-atom crystal), however, the image is one of the electrical characteristics of the surface rather than of the optical characteristics as with a conventional, optical microscope.
Although the aforementioned characteristic of the scanning tunnel microscope does not necessarily represent a disadvantage, since excellent data about the surface quality, particularly with respect to defects, etc., can be derived from the electrical characteristics of the surface, application of this instrument is limited to electrically conductive surfaces. There have been experiments to force the surface of originally nonconductive materials--particularly of organic substances--to be conductive by immersion in a conductive liquid, but this has obvious disadvantages, especially when the surface to be examined is intended as an insulation and must retain this function, as for example an insulating oxide layer on a silicon surface.
A further drawback of the scanning tunnel microscope is the fact that the electron current required for measurement can alter the surface, e.g., the molecular structure can be split by the current. This may be a desirable effect when used as a tool for surface treatment, but it is an undesirable effect for a nondestructive measurement.
Based on the aforementioned problems, as early as 1985--also in the IBM Ruschlikon Research Laboratory--the atomic force microscope (AFM) was proposed. In this AFM disclosed by Binnig, Quate, and Gerbar in Phys. Rev. Letters, 56, 930 (1986), the repulsive force of a nonconductive tip (e.g., diamond) with respect to the surface is measured by depressing the tip with an elastic force. Many of the disadvantages of the scanning tunnel microscope have been overcome by this sensor arrangement. In particular, it is also possible to image nonconductive surfaces, and the repulsive force of a nonconductive surface corresponds more readily to the optical image than the plane of equal tunnel probability based on the tunnel effect.
However, a significant disadvantage of the scanning tunnel microscope is not eliminated by the AFM and in certain circumstances is even aggravated. The measurement of the repulsive force itself is conducive to damaging the surface--e.g., by displacing the surface atoms to lower levels, by contamination and deposition of residual impurities in the vicinity of the surface, etc.
Based on this problem, the laser force microscope was developed at the IBM Yorktown Laboratory. Unlike the AFM, this type of force microscope employs far-reaching "weak" interactions, such as the van der Waals forces. The forces, and consequently the surface stress, are lower than in the AFM by several orders of magnitude.
The actual measurement principle of the laser force microscope lies in exciting the probe, e.g., using a piezoelectric transducer, at or near a resonant frequency of the probe and measuring the forces arising from the surface by observing their influence on the resonant frequency.
The measurement can be accomplished either by tracking the excitation frequency to the effective resonant frequency and using the frequency shift as a measure for the force exerted on the probe by the surface (frequency modulation) or by using the reduction of the amplitude directly as a measure for the frequency shift.
While thin tungsten wires were employed in the initial versions of laser force microscopes, silicon needles were also developed whose vibration is then measured with heterodyne laser measurement methods.
This measurement method, in use for several years now, has the disadvantage, however, that the measurement detects only one component of the force, that is, the projection of the force vector onto the axis of the induced vibration. When the force vector is not orthogonal to the surface, for example because the probe is positioned in and at the edge of a depression, the measurement is subject to inherent error.